Re: vhost-user protocol feature negotiation

[Michael S. Tsirkin]

On Thu, Aug 06, 2020 at 11:24:59AM +0000, Alyssa Ross wrote:
> "Michael S. Tsirkin" <mst@redhat.com> writes:
> 
> > On Thu, Aug 06, 2020 at 08:59:09AM +0000, Alyssa Ross wrote:
> >> "Michael S. Tsirkin" <mst@redhat.com> writes:
> >> 
> >> > On Wed, Aug 05, 2020 at 03:13:06PM +0000, Alyssa Ross wrote:
> >> >> Quoting from the definition of VHOST_USER_SET_PROTOCOL_FEATURES in
> >> >> vhost-user.rst:
> >> >> 
> >> >> >   Only legal if feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` is 
> >> >> > present in
> >> >> >   ``VHOST_USER_GET_FEATURES``.
> >> >> > 
> >> >> > .. Note::
> >> >> >    Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must support
> >> >> >    this message even before ``VHOST_USER_SET_FEATURES`` was called.
> >> >> 
> >> >> To me, this could mean either of two things:
> >> >> 
> >> >> (1) If VHOST_USER_F_PROTOCOL_FEATURES hasn't been set, upon receiving
> >> >>     VHOST_USER_SET_PROTOCOL_FEATURES, a backend should enable the
> >> >>     protocol features immediately.
> >> >> 
> >> >> (2) If VHOST_USER_F_PROTOCOL_FEATURES hasn't been set, upon receiving
> >> >>     VHOST_USER_SET_PROTOCOL_FEATURES, a backend should store those
> >> >>     feature bits, but not actually consider them to be enabled until
> >> >>     after VHOST_USER_SET_FEATURES has been received (presumably
> >> >>     containing VHOST_USER_F_PROTOCOL_FEATURES).
> >> >> 
> >> >> The reason I bring this up is that QEMU appears to interpret it as (1),
> >> >> while the vhost-user-net backend in Intel's cloud-hypervisor[1]
> >> >> interprets it as (2).  So I'm looking for a clarification.
> >> >> 
> >> >> [1]: https://github.com/cloud-hypervisor/cloud-hypervisor
> >> >> 
> >> >> Thanks in advance.
> >> >
> >> >
> >> > IMHO the intent was this: VHOST_USER_F_PROTOCOL_FEATURES bit in
> >> > VHOST_USER_GET_FEATURES means that qemu can send
> >> > VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES.
> >> >
> >> > With most feature bits in VHOST_USER_GET_FEATURES, the
> >> > specific functionality needs to only be enabled after
> >> > VHOST_USER_SET_FEATURES.
> >> >
> >> > However, this is for functionality dealing with guest activity.
> >> > VHOST_USER_SET_PROTOCOL_FEATURES has nothing to do with guest directly,
> >> > it's about negotiation between qemu and backend: it is only in
> >> > VHOST_USER_GET_FEATURES for the reason that this is the only message
> >> > (very) old backends reported.  Thus, the backend should not check
> >> > whether VHOST_USER_SET_FEATURES sets VHOST_USER_F_PROTOCOL_FEATURES,
> >> > instead it should simply always be ready to receive
> >> > VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES.
> >> >
> >> > Backend that isn't always ready to handle
> >> > VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES
> >> > should not set VHOST_USER_F_PROTOCOL_FEATURES in
> >> > VHOST_USER_GET_FEATURES.
> >> 
> >> Thanks for the explanation.  That matches what I had in mind with (1).
> >> 
> >> > This appears to be closer to (1), but if qemu can't distinguish
> >> > then we don't care, right? For example, VHOST_USER_PROTOCOL_F_REPLY_ACK
> >> > enables acks on arbitrary messages. Does the backend in question
> >> > ignore the affected bit until SET_FEATURES? If yes won't this
> >> > make qemu hang?
> >> 
> >> Yes.  That was my motivation for asking what the correct behaviour was,
> >> so that I could fix the incorrect one. :)  I suspect that up to this point,
> >> the cloud-hypervisor vhost-user-net backend has only been used with
> >> cloud-hypervisor, and so this incompatibilty with QEMU was not noticed.
> >> 
> >> > How would you suggest clarifying the wording?
> >> 
> >> Do you think this communicates everything required?
> >> 
> >> ---
> >> diff --git i/docs/interop/vhost-user.rst w/docs/interop/vhost-user.rst
> >> index 10e3e3475e..72724d292a 100644
> >> --- i/docs/interop/vhost-user.rst
> >> +++ w/docs/interop/vhost-user.rst
> >> @@ -854,9 +854,8 @@ Master message types
> >>    ``VHOST_USER_GET_FEATURES``.
> >>  
> >>  .. Note::
> >> -   Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must
> >> -   support this message even before ``VHOST_USER_SET_FEATURES`` was
> >> -   called.
> >> +   ``VHOST_USER_F_PROTOCOL_FEATURES`` does not need to be acknowledged
> >> +   with ``VHOST_USER_SET_FEATURES``.
> >>  
> >>  ``VHOST_USER_SET_PROTOCOL_FEATURES``
> >>    :id: 16
> >
> > Hmm I find this confusing. I think it's a good policy to ask qemu to
> > acknowledge it. It's just that the client should not wait for
> > VHOST_USER_SET_FEATURES before handling VHOST_USER_SET_PROTOCOL_FEATURES
> > or VHOST_USER_GET_PROTOCOL_FEATURES.
> 
> To me, it's confusing that a frontend is expected to ack
> VHOST_USER_F_PROTOCOL_FEATURES even though the ack can't have any effect
> (because VHOST_USER_GET_PROTOCOL_FEATURES and
> VHOST_USER_SET_PROTOCOL_FEATURES both have to work even if the ack
> hasn't been received yet).
> 
> But, if the frontend is supposed to ack anyway, how about:
> 
> Signed-off-by: Alyssa Ross <hi@alysas.is>
> 
> ---
> diff --git i/docs/interop/vhost-user.rst w/docs/interop/vhost-user.rst
> index 10e3e3475e..bc78c9947f 100644
> --- i/docs/interop/vhost-user.rst
> +++ w/docs/interop/vhost-user.rst
> @@ -854,9 +854,9 @@ Master message types
>    ``VHOST_USER_GET_FEATURES``.
>  
>  .. Note::
> -   Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must
> -   support this message even before ``VHOST_USER_SET_FEATURES`` was
> -   called.
> +   While QEMU should acknowledge ``VHOST_USER_F_PROTOCOL_FEATURES``, a
> +   backend must allow ``VHOST_USER_GET_PROTOCOL_FEATURES`` even if
> +   ``VHOST_USER_F_PROTOCOL_FEATURES`` has not been acknowledged yet.
>  
>  ``VHOST_USER_SET_PROTOCOL_FEATURES``
>    :id: 16
> @@ -869,8 +869,12 @@ Master message types
>    ``VHOST_USER_GET_FEATURES``.
>  
>  .. Note::
> -   Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must support
> -   this message even before ``VHOST_USER_SET_FEATURES`` was called.
> +   While QEMU should acknowledge ``VHOST_USER_F_PROTOCOL_FEATURES``, a
> +   backend must allow ``VHOST_USER_SET_PROTOCOL_FEATURES`` even if
> +   ``VHOST_USER_F_PROTOCOL_FEATURES`` has not been acknowledged yet.
> +   The backend must not wait for ``VHOST_USER_SET_FEATURES`` before
> +   enabling protocol features requested with
> +   ``VHOST_USER_SET_PROTOCOL_FEATURES``.
>  
>  ``VHOST_USER_SET_OWNER``
>    :id: 3



That looks good to me.
Pls post a patch on list, preferably after qemu is released.